Busbar Ampacity
- Thread starter jman34
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- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
Re: Busbar Ampacity
It's not in the NEC. I thought the only source of that type of information was the manufacturer of the bus bar. Is there a calculation method?
It's not in the NEC. I thought the only source of that type of information was the manufacturer of the bus bar. Is there a calculation method?
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Re: Busbar Ampacity
There are "rules of thumb" that have been used such as 1000A per square inch open or 800A enclosed for 40C ambient, 30C rise, and 98% conductivity but they are not accurate.
In reality the ampacity of a single bus bar is not a constant per square inch. Surface area for heat disipation is very important as well as proximity to other bus bars. For example:
single bar
1/4 x 2" - 647A
1/2 x 2" - 990A
1/4 x 4" - 1220A
two parallel bars seperated by 2 3/8" (like ||)
1/4 x 2" - 1178A
1/2 x 2" - 1802
1/4 x 4" - 2220
two parallel bars seperated by 1/4" (like ||)
1/4 x 2" - 1020A
1/2 x 2" - 1560
1/4 x 4" - 1925
Contact any bus bar manufacturer for actual values. My examples are from a chart dated 1964.
There are "rules of thumb" that have been used such as 1000A per square inch open or 800A enclosed for 40C ambient, 30C rise, and 98% conductivity but they are not accurate.
In reality the ampacity of a single bus bar is not a constant per square inch. Surface area for heat disipation is very important as well as proximity to other bus bars. For example:
single bar
1/4 x 2" - 647A
1/2 x 2" - 990A
1/4 x 4" - 1220A
two parallel bars seperated by 2 3/8" (like ||)
1/4 x 2" - 1178A
1/2 x 2" - 1802
1/4 x 4" - 2220
two parallel bars seperated by 1/4" (like ||)
1/4 x 2" - 1020A
1/2 x 2" - 1560
1/4 x 4" - 1925
Contact any bus bar manufacturer for actual values. My examples are from a chart dated 1964.
- Location
- Mission Viejo, CA
- Occupation
- Professional Electrical Engineer
Re: Busbar Ampacity
Both Charlie and Jim are right. If you have seen a formula, it is most likely a "rule of thumb."
The standard for bus is neither UL nor NEMA, but ANSI/IEEE C37.23. The "formula" for calculating the ampacity is right up there with Neher-McGrath calcs. I don't have a ready copy, but as I recall, there are approximately 7-10 variables, including the "shape" of the bar itself.
In general, the ultimate rating is determined by a temperature rise test against a specific standard rating. In other words, if a continuous 1200A through a particular configuration, at specified ambient temperature, doesn't result in a rise above 90C, then it is a 1200A bus.
Both Charlie and Jim are right. If you have seen a formula, it is most likely a "rule of thumb."
The standard for bus is neither UL nor NEMA, but ANSI/IEEE C37.23. The "formula" for calculating the ampacity is right up there with Neher-McGrath calcs. I don't have a ready copy, but as I recall, there are approximately 7-10 variables, including the "shape" of the bar itself.
In general, the ultimate rating is determined by a temperature rise test against a specific standard rating. In other words, if a continuous 1200A through a particular configuration, at specified ambient temperature, doesn't result in a rise above 90C, then it is a 1200A bus.
- Location
- Illinois
- Occupation
- retired electrician
Re: Busbar Ampacity
It does appear in the code in the Auxiliary Gutter Article.
It does appear in the code in the Auxiliary Gutter Article.
Don
pault587
Member
- Location
- Grand Rapids, MI
Re: Busbar Ampacity
Be careful when talking about buss bar. I ran in to trouble last year using these rules of thumb that are mentioned above. If you are delaying with DC you probably won't have any trouble but with AC you run into things like skin effect, and proximity problems from induced inductance. I work on plating and anodizing lines and this where I ran into a problem. These problems may not show in other applications but since you did not mention the application I thought I would throw it out there.
Be careful when talking about buss bar. I ran in to trouble last year using these rules of thumb that are mentioned above. If you are delaying with DC you probably won't have any trouble but with AC you run into things like skin effect, and proximity problems from induced inductance. I work on plating and anodizing lines and this where I ran into a problem. These problems may not show in other applications but since you did not mention the application I thought I would throw it out there.
- Location
- Mission Viejo, CA
- Occupation
- Professional Electrical Engineer
Re: Busbar Ampacity
Great catch Don. I was aware of the 1000A/in2 as a "rule of thumb" but I was totally unaware it was codified.
I suppose it is a sufficiently conservative figure that it can be used as a field application even without specifying ambient conditions. However, Jim's table listed a "low" of 990A/in2. If the bar were truly 1"x1" the ampacity would be even lower, although I couldn't say how much.
Refering to 310.15 isn't enough to adjust it either since "bare copper bars" isn't included in the tables. I think I still prefer having a manufacturer's tested or certified rating after I've spec'ed the "conditions of use."
Great catch Don. I was aware of the 1000A/in2 as a "rule of thumb" but I was totally unaware it was codified.
I suppose it is a sufficiently conservative figure that it can be used as a field application even without specifying ambient conditions. However, Jim's table listed a "low" of 990A/in2. If the bar were truly 1"x1" the ampacity would be even lower, although I couldn't say how much.
Refering to 310.15 isn't enough to adjust it either since "bare copper bars" isn't included in the tables. I think I still prefer having a manufacturer's tested or certified rating after I've spec'ed the "conditions of use."
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Re: Busbar Ampacity
Most major equipment manufacturers do not use current density for sizing switchboard and panel bussing. They design to the heat rise tests of UL, so totally enclosed bus will be sized differently than ventilated bus. Medium voltage switchgear (usually designed to ANSI standards) and custom equipment are more likely to use current density.
Most major equipment manufacturers do not use current density for sizing switchboard and panel bussing. They design to the heat rise tests of UL, so totally enclosed bus will be sized differently than ventilated bus. Medium voltage switchgear (usually designed to ANSI standards) and custom equipment are more likely to use current density.
ron
Senior Member
- Location
- New York, 40.7514,-73.9925
Re: Busbar Ampacity
Here's a good link, and for obvious reasons only applies to copper bus.
http://www.copper.org/applications/busbar/homepage.html
and a link within the previous:
http://www.copper.org/applications/busbar/ampacity/busT1.htm
[ September 17, 2004, 11:20 AM: Message edited by: ron ]
Here's a good link, and for obvious reasons only applies to copper bus.
http://www.copper.org/applications/busbar/homepage.html
and a link within the previous:
http://www.copper.org/applications/busbar/ampacity/busT1.htm
[ September 17, 2004, 11:20 AM: Message edited by: ron ]
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